Such an amplifier used in a line circuit is known from the U.S. Pat. No. 4,041,252.
Such line circuits are used on the basis of "one per subscriber's line" in subscriber's exchanges based on time-division multiplex or small signal spatial networks. Therefore, the aim is to keep the construction cost of these line circuits as low as possible. As very large numbers are required, this requires the highest possible degree of circuit integration. The amplifier produces the alternating current as well as the direct current for the subscriber's lines. The value of the line current depends on the length of the subscriber's line. In order to be capable of supplying sufficient line current with very long lines, the voltage of the voltage source is normally 48 or 60 Volt. This means that the line current in the case of very short lines or a short circuit in the subscriber's line can be very large. Telephone administrations therefore prescribe a 400 to 800 Ohm supply resistor arranged in series with the line, by means of which the maximum line current is limited to 100 to 200 mA. Since a subscriber's line is a two-wire transmission line, each line circuit comprises for each wire an impedance and an amplifier which are constructed so that the line circuits are of a symmetrical construction. The supply resistance is formed by the two impedances, each impedance having a value of 200 to 400 Ohm. These impedances also constitute the terminating impedances of the subscriber's line. To prevent the impedances from converting common mode signals on the subscriber's line into differential mode signals, the administrations require an accuracy of approximately 0.1% for these impedances. The maximum line current, limited by means of the supply resistor, is still so high that the output transistor of the amplifier dissipates a relatively high power. So far this dissipation has been an obstacle in the endeavours to implement these amplifiers in integrated form in a cheap manner with sufficient reliability and long life for telephony purposes, for example 20 years.
It is an object of the invention at least to mitigate this drawback and to achieve an amplifier whose dissipation in the semiconductor components is low, so that it is easily integrable and satisfies the above-mentioned requirements.